A random-access (RA) procedure is a key function in a cellular system. For example, in Long Term Evolution (LTE), a user equipment (UE) (which may be referred to interchangeably as a mobile terminal or wireless device) may access the network by participating in a random-access procedure with a transmission and reception point (TRP), such as a base station, access node, or access point. The UE that would like to access the network initiates the random-access procedure by transmitting a preamble (Msg1) in the uplink on the Physical Random-Access Channel (PRACH). The TRP receiving the preamble and detecting the random-access attempt will respond in the downlink by transmitting a random-access response (RAR, Msg2). The RAR carries an uplink scheduling grant for the UE to continue the procedure by transmitting a following subsequent message in the uplink (Msg3) for terminal identification. A similar procedure is envisioned for NR, see an illustration in FIG. 1 (i.e., FIG. 1A-1B).
In LTE, the RAR is transmitted over Physical Downlink Control CHannel (PDCCH)/Physical Downlink Shared Channel (PDSCH), similar to normal data transmission. More specifically, the RAR is sent on the Downlink Shared CHannel (DL-SCH, such as PDSCH) and indicated on the PDCCH using an identity reserved for RAR, the Random Access Radio-Network Temporary Identifier (RA-RNTI) (instead of a Cell Radio-Network Temporary Identifier (C-RNTI) as used for already connected UEs). The RAR message contains, among other items, the detected PRACH preamble index as well as a temporary UE identity, Temporary C-RNTI (TC-RNTI), to be used for further communication between the UE and the network, until a C-RNTI has been assigned to the UE. RAR messages to multiple UEs can be combined into a single transmission over DL-SCH. See e.g., Dahlman et al., “3G evolution: HSPA and LTE for mobile broadband” for more information.
In LTE, the UE receiving the RAR can assume that the RAR is well synchronized with the LTE synchronization signal (called primary synchronization signal (PSS)/secondary synchronization signal (SSS)) and the receiver can hence base its time and frequency synchronization on PSS/SSS and only use the cell-specific reference signals (CRS) in the RAR transmission for the detailed channel estimate (phase estimate), similar to the case of normal data transmission.
In contrast, in NR, a UE might have to be able to receive RAR in a somewhat large timing uncertainty interval relative to the synchronization signal (SS). Reasons for large timing offset between received SS and RAR (using UE perspective) may be e.g., [R1-167060, R1-1609673]:
1. Narrowband and Low Signal-to-Noise Ratio (SNR) for SS                only enough downlink timing accuracy for small broadcast        Without beamforming of SS, it might be received with low SNR        
2. Different Nodes for SS and RAR                Propagation: 1 km corresponds to a round trip time of 3.3 μs. This should be compared to a cyclic prefix of approximately 4 μs        
3. Non-Synchronized Nodes                Or “loosely” synchronized nodes        With SFN (Single Frequency Network) type of SS transmission        
For these reason, it is proposed to have a self-contained RAR in NR [R1-167060, R1-1609673], i.e., the RAR contains its own dedicated synchronization signal as illustrated in FIG. 2. Each box would typically represent one Orthogonal Frequency Domain Multiplexing (OFDM) symbol (or similar, depending on modulation scheme used), and the dashed boxes are optional symbols if needed to encompass the RAR payload.
If there is only one data signal (DS) symbol (which may often be enough), the synchronization signal reference signal (RS) can likely be used also as a phase reference for channel estimation. If there are multiple DS symbols, there may be a need for additional dedicated reference symbols (DMRS) in the DS part, but those would then typically need to occupy just a subset of the subcarriers.
If the RAR signal, as received at the UE location, is not sufficiently closely aligned with the current downlink (DL) time reference of the UE, the DS field cannot be reliably demodulated and decoded. The aim of the RS field is then to allow establishing an improved time reference, as well as a frequency reference, for placing the fast Fourier transform (FFT) window for DS symbol demodulation. Synchronization search is typically implemented as time-domain correlation of the received sample sequence with reference sequences representing the transmitted RS signal. The reference timing is inferred when a sufficiently high correlation peak is detected. The time-domain correlation is a relatively expensive operation and needs to be carried out individually for each possible RS sequence, so RAR search complexity is proportional to the number of possible RS sequences the UE needs to account for. For the UE, the best configuration is therefore a single RS that applies to any RAR transmission.
The random-access procedure (Msg1-3) employing the RAR design from FIG. 2 is illustrated along with timings in FIG. 3.